Proper function of the human brain requires precise regulation during development and maintenance throughout adulthood. For neurons to function and signal properly, synaptic contacts between them are made using axons and dendrites that grow and branch in response to environmental cues. Numerous psychiatric disorders, such as schizophrenia, autism, and mood disorders exhibit reduced and aberrant connections between neurons. The RhoA GTPase (Rho) is an essential regulator of the cytoskeleton, and changes in its activity lead to alterations in the connections extension and retractions of axons and dendrites. Rho activity is controlled by activating guanine nucleotide exchange factors (GEFs) and inhibitory GTPase activating proteins (GAPs). One such regulator, p190RhoGAP (p190), plays a key role in inhibiting Rho and preventing the retraction of neurites and likely maintaining synaptic integrity. p190 is phosphorylated and activated by the non-receptor tyrosine kinase Arg (Abl-related gene) in response to integrin-mediated adhesion. Arg also plays a crucial role in neuronal maintenance, as neurons lacking Arg display reduced a drastic reduction in dendrite number and branching. Despite the correlative evidence linking Arg, p190, and Rho to neuronal structure and function, there is little mechanistic detail directly relating the proteins. The experiments I propose here will shed light on the pathway linking Arg-mediated p190 inhibition of Rho to changes in the cytoskeleton and morphology of fibroblasts and neurons. In the first aim I will define the mechanism of Arg activation in response to integrin-mediated adhesion. In the second aim I will develop probes necessary for observing real time changes in p190 activation and localization, Rho activity, and cytoskeletal and cell morphological changes necessary to directly link these events in this pathway. In the third aim, using the probes developed in aim 2, I will simultaneously monitor how the localized activity of p190 and its inhibition of Rho affect cytoskeletal and morphological dynamics in fibroblasts and neurons. Together these studies will provide much needed mechanistic detail relating Arg- mediated p190 inhibition of Rho with changes in the cytoskeleton and cell morphology. This will provide evidence highlighting the roles Arg, p190, and Rho play in maintenance and disruption of neuronal morphology. The long term goal is to reveal novel strategies for manipulating Rho activity to prevent the deleterious effects its dysregulation has on dendritic structure in hopes of preventing or treating psychiatric disorders, such as schizophrenia, autism, and mood disorders. [unreadable] [unreadable]